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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="fusion.introduction"></a><a class="link" href="introduction.html" title="Introduction">Introduction</a>
</h2></div></div></div>
<p>
      An advantage other languages such as Python and Lisp/ Scheme, ML and Haskell,
      etc., over C++ is the ability to have heterogeneous containers that can hold
      arbitrary element types. All the containers in the standard library can only
      hold a specific type. A <code class="computeroutput"><span class="identifier">vector</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>
      can only hold <code class="computeroutput"><span class="keyword">int</span></code>s. A <code class="computeroutput"><span class="identifier">list</span><span class="special">&lt;</span><span class="identifier">X</span><span class="special">&gt;</span></code> can
      only hold elements of type <code class="computeroutput"><span class="identifier">X</span></code>,
      and so on.
    </p>
<p>
      True, you can use inheritance to make the containers hold different types,
      related through subclassing. However, you have to hold the objects through
      a pointer or smart reference of some sort. Doing this, you'll have to rely
      on virtual functions to provide polymorphic behavior since the actual type
      is erased as soon as you store a pointer to a derived class to a pointer to
      its base. The held objects must be related: you cannot hold objects of unrelated
      types such as <code class="computeroutput"><span class="keyword">char</span></code>, <code class="computeroutput"><span class="keyword">int</span></code>, <code class="computeroutput"><span class="keyword">class</span>
      <span class="identifier">X</span></code>, <code class="computeroutput"><span class="keyword">float</span></code>,
      etc. Oh sure you can use something like <a href="http://www.boost.org/libs/any" target="_top">Boost.Any
      Library</a> to hold arbitrary types, but then you pay more in terms of
      runtime costs and due to the fact that you practically erased all type information,
      you'll have to perform dangerous casts to get back the original type.
    </p>
<p>
      The <a href="http://www.boost.org/libs/tuple" target="_top">Boost.Tuple</a> library
      written by <a href="http://www.boost.org/people/jaakko_jarvi.htm" target="_top">Jaakko
      Jarvi</a> provides heterogeneous containers in C++. The <code class="computeroutput"><span class="identifier">tuple</span></code>
      is a basic data structure that can hold heterogeneous types. It's a good first
      step, but it's not complete. What's missing are the algorithms. It's nice that
      we can store and retrieve data to and from tuples, pass them around as arguments
      and return types. As it is, the <a href="http://www.boost.org/libs/tuple" target="_top">Boost.Tuple</a>
      facility is already very useful. Yet, as soon as you use it more often, usage
      patterns emerge. Eventually, you collect these patterns into algorithm libraries.
    </p>
<p>
      Hmmm, kinda reminds us of STL right? Right! Can you imagine how it would be
      like if you used STL without the algorithms? Everyone will have to reinvent
      their own <span class="emphasis"><em>algorithm</em></span> wheels.
    </p>
<p>
      Fusion is a library and a framework similar to both <a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>
      and the boost <a href="http://www.boost.org/libs/mpl" target="_top">MPL</a>. The structure
      is modeled after <a href="http://www.boost.org/libs/mpl" target="_top">MPL</a>, which
      is modeled after <a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>.
      It is named "fusion" because the library is reminiscent of the "fusion"
      of compile time meta-programming with runtime programming. The library inherently
      has some interesting flavors and characteristics of both <a href="http://www.boost.org/libs/mpl" target="_top">MPL</a>
      and <a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>.
      It lives in the twilight zone between compile time meta-programming and run
      time programming. <a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>
      containers work on values. MPL containers work on types. Fusion containers
      work on both types and values.
    </p>
<p>
      Unlike <a href="http://www.boost.org/libs/mpl" target="_top">MPL</a>, Fusion algorithms
      are lazy and non sequence-type preserving. What does that mean? It means that
      when you operate on a sequence through a Fusion algorithm that returns a sequence,
      the sequence returned may not be of the same class as the original. This is
      by design. Runtime efficiency is given a high priority. Like <a href="http://www.boost.org/libs/mpl" target="_top">MPL</a>,
      and unlike <a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>,
      fusion algorithms are functional in nature such that algorithms are non mutating
      (no side effects). However, due to the high cost of returning full sequences
      such as vectors and lists, <span class="emphasis"><em>Views</em></span> are returned from Fusion
      algorithms instead. For example, the <a class="link" href="algorithm/transformation/functions/transform.html" title="transform"><code class="computeroutput"><span class="identifier">transform</span></code></a> algorithm does not actually
      return a transformed version of the original sequence. <a class="link" href="algorithm/transformation/functions/transform.html" title="transform"><code class="computeroutput"><span class="identifier">transform</span></code></a> returns a <a class="link" href="view/transform_view.html" title="transform_view"><code class="computeroutput"><span class="identifier">transform_view</span></code></a>. This view holds a
      reference to the original sequence plus the transform function. Iteration over
      the <a class="link" href="view/transform_view.html" title="transform_view"><code class="computeroutput"><span class="identifier">transform_view</span></code></a>
      will apply the transform function over the sequence elements on demand. This
      <span class="emphasis"><em>lazy</em></span> evaluation scheme allows us to chain as many algorithms
      as we want without incurring a high runtime penalty.
    </p>
<p>
      The <span class="emphasis"><em>lazy</em></span> evaluation scheme where algorithms return views
      allows operations such as <a class="link" href="algorithm/transformation/functions/push_back.html" title="push_back"><code class="computeroutput"><span class="identifier">push_back</span></code></a> to be totally generic. In
      Fusion, <a class="link" href="algorithm/transformation/functions/push_back.html" title="push_back"><code class="computeroutput"><span class="identifier">push_back</span></code></a> is actually a generic algorithm
      that works on all sequences. Given an input sequence <code class="computeroutput"><span class="identifier">s</span></code>
      and a value <code class="computeroutput"><span class="identifier">x</span></code>, Fusion's <a class="link" href="algorithm/transformation/functions/push_back.html" title="push_back"><code class="computeroutput"><span class="identifier">push_back</span></code></a> algorithm simply returns
      a <a class="link" href="view/joint_view.html" title="joint_view"><code class="computeroutput"><span class="identifier">joint_view</span></code></a>:
      a view that holds a reference to the original sequence <code class="computeroutput"><span class="identifier">s</span></code>
      and the value <code class="computeroutput"><span class="identifier">x</span></code>. Functions
      that were once sequence specific and need to be implemented N times over N
      different sequences are now implemented only once.
    </p>
<p>
      Fusion provides full round compatibility with <a href="http://www.boost.org/libs/mpl" target="_top">MPL</a>.
      Fusion sequences are fully conforming <a href="http://www.boost.org/libs/mpl" target="_top">MPL</a>
      sequences and <a href="http://www.boost.org/libs/mpl" target="_top">MPL</a> sequences
      are fully compatible with Fusion. You can work with Fusion sequences on <a href="http://www.boost.org/libs/mpl" target="_top">MPL</a> if you wish to work solely
      on types <a href="#ftn.fusion.introduction.f0" class="footnote" name="fusion.introduction.f0"><sup class="footnote">[1]</sup></a>. In <a href="http://www.boost.org/libs/mpl" target="_top">MPL</a>, Fusion
      sequences follow <a href="http://www.boost.org/libs/mpl" target="_top">MPL</a>'s sequence-type
      preserving semantics (i.e. algorithms preserve the original sequence type.
      e.g. transforming a vector returns a vector). You can also convert from an
      <a href="http://www.boost.org/libs/mpl" target="_top">MPL</a> sequence to a Fusion
      sequence. For example, there are times when it is convenient to work solely
      on <a href="http://www.boost.org/libs/mpl" target="_top">MPL</a> using pure <a href="http://www.boost.org/libs/mpl" target="_top">MPL</a> sequences, then, convert them
      to Fusion sequences as a final step before actual instantiation of real runtime
      objects with data. You have the best of both worlds.
    </p>
<div class="footnotes">
<br><hr style="width:100; text-align:left;margin-left: 0">
<div id="ftn.fusion.introduction.f0" class="footnote"><p><a href="#fusion.introduction.f0" class="para"><sup class="para">[1] </sup></a>
        Choose <a href="http://www.boost.org/libs/mpl" target="_top">MPL</a> over fusion
        when doing pure type calculations. Once the static type calculation is finished,
        you can instantiate a fusion sequence (see <a class="link" href="container/conversion/functions.html" title="Functions">Conversion</a>)
        for the runtime part.
      </p></div>
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      Dan Marsden, Tobias Schwinger<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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